The present invention relates to magnetic recording and more particularly to methods and means for controlling the bias as supplied to the record head during recording.
When tape or rather wire recording was originally invented, no bias was used in the recording head, i.e. only the magnetic signals as produced by the electric sound signals were transferred to the magnetic medium as it moved past the recording head. Later on it was discovered that the quality of the recording was improved when a constant DC bias was applied to the recording head, because the AC sound signal could then cause the medium to be magnetized without distortion due to the hysteresis phenomena. Still later it was discovered that a constant AC bias of a relatively high frequency was much better, because it did away with a noise problem caused by the DC bias and showed additional advantages especially with respect to the intensity of the recorded signal. Thus, with the use of an AC bias it is possible to create effective magnetic impressions of sound signals on modern tapes having a very thin layer of magnetic medium and being moved relatively slowly past the recorder head.
It has been found, however, that there are some problems connected with the adjustment of the intensity of the AC bias to an optimum, because it seems to be true that in the low end of the sound frequency spectrum the optimal bias intensity is higher than what is optimal for the frequencies of the high end of the spectrum. It is normal practice, therefore, to select a bias intensity which is optimal to a broad middle range of the sound frequency spectrum, while non-optimal to both the low and the high frequencies, whereby the magnetizing capacity of the tape is not fully utilized for these frequencies.
Sporadic attempts have been made to solve this problem, yet so far without real success. The attempts have concentrated on a dynamic or variable bias solution, whereby sensor means are used in an open loop system for steadily sensing the proportion of high or low frequencies in the sound signal and for controlling the intensity of the applied bias accordingly, i.e. such that the bias intensity is reduced whenever the sound signal has a relatively large proportion of high frequencies and/or increased when it is the low frequency proportion which is prevailing. These known solutions have resulted in certain improvements, but the control has not been fully stable, and the control criteria have not been particularly well defined, as they are mostly empirically based.
Thus, as known to those skilled in the art, any change of the bias intensity will affect various tape characteristics such as the sensitivity and the magnetizing capacity of the magnetic medium generally for all sound frequencies, and though a dynamically varying bias may well to some degree optimize the recording of the low or high sound frequencies, respectively, it will nevertheless be liable to disturb the correct uniform recording of the entire remaining major sound frequency range. Also for this reason the said known variable bias intensity systems have not brought about any real solution to the problem of providing for a generally optimized bias.